This application relates to a pressure roll assembly for applying pressure to sheet material articles being driven by a belt conveyor. It relates particularly to a pressure roll assembly which is designed to precisely control the pressure on the sheet material articles, in order to apply enough pressure to hold the sheet material articles on the conveyor, and yet to eliminate or substantially reduce noticeable ink tracking on sheet material articles having freshly printed ink on their upward facing sides.
In the sheet handling arts there are various instances in which sheet material articles are driven by means of a conveyor which is formed by one or more moving belts. In such systems it is often desirable to provide a pressure roll assembly forming a nip with the belt conveyor. The pressure roll assembly functions to apply pressure forces to the sheet material articles to maintain positive engagement of the sheet material articles with the conveyor belts, in order to assure precise driving of the sheet material articles by the belts.
One type of known prior art pressure roll assembly includes a longitudinally extending support rod disposed in fixed relation above and longitudinally with respect to the belt conveyor and a rockable support shaft extending transversely to the support rod. One end of a leaf spring is connected with the support shaft, and the other end of the leaf spring journals a rotatable pressure roll whose axis of rotation is parallel to the rotatable axle. The pressure roll is freely rotatable about its axis and includes a knurled outer surface. A releasable clamp fixes the position of the support shaft relative to the support rod as well as its angle of rock. In order to adjust the pressure in the nip between the roll and the belt it is necessary to release the clamp and manually rock the rotatable axle to the desired position. The clamp is then engaged to lock the roll in the adjusted position.
Another apparatus for applying pressure to sheet material articles is illustrated in U.S. Pat. No. 3,741,536. This patent illustrates a support disposed above the conveyor belts, and the support includes means for freely supporting a series of balls. An adjustment screw arrangement is connected with the ball support and is designed to adjust the position of the ball support above the conveyor.
There are various instances in the sheet handling art in which precise and fine adjustment of the tension in the nip between the pressure roll and the conveyor belts is required. One circumstance which requires precise adjustment occurs when sheets having freshly printed material are fed onto the conveyor in a "face up" condition. This situation may occur in sheet duplicating mechanisms. When sheets are fed in a "face up" condition an imprecisely tensioned roller may pick up ink and deposit it on areas of the sheets. This condition is known to those in the art as tracking. The system of U.S. Pat. No. 3,741,536 is not as capable of reducing or minimizing tracking as the present invention, because of the fact that the balls have smooth surfaces (which in itself tends to pick up substantial ink). In addition, with heavy (thick) sheet material articles, the balls may bounce considerably when they are engaged by the sheet stock and hence fail to feed the sheet reliably. Mainly, however, precise adjustment of pressure is not possible because the weight of the balls is fixed, and the balls are essentially resting with their full weight on the sheet or else are raised so as to have zero or nominal sheet contact.
In pressure roll assemblies which employ a rockable support shaft which is manually rocked to set the roll pressure, it is very difficult to precisely adjust the pressure of the roll. While a knurled roll surface, per se, may produce less tracking than a spherical ball surface, the imprecision in obtaining fine adjustment of this mechanism makes it very difficult to avoid tracking problems.
Another reason requiring very precise control of the pressure in the nip between the roll and the belt occurs when the thickness of the stock being driven by the conveyor varies. Heavy, thick, stock can result in considerable bouncing of the balls in the system of U.S. Pat. No. 3,741,536.
Another prior art pressure roll construction is illustrated in U.S. Pat. No. 1,299,386. According to this disclosure a pressure roll or roller is supported at the end of a curved spring member which is fixedly connected to a fixed support. Movement of the roller to adjust pressure is provided by movement of a screw which engages the curved spring member adjacent the roller, so that the roller pressure setting is essentially a direct function of the roller position as determined by the screw setting.